1. Field of the Invention
The present invention relates to a wireless electrical charging apparatus adopting a noncontact power supplying method allowing power to be transferred to a battery employed in an electric car or the like as power to be supplied to the battery, and relates to a wireless electrical charging system.
2. Description of the Related Art
Typically, a battery employed in an electric car is electrically charged by adoption of a general method by which a cable of an electrical charging stand is plugged into the battery in order to electrically charge the battery.
However, the amount of transferred power is large, requiring a thick and heavy cable to be plugged into the battery and connected to the battery. In addition, if the operation to electrically charge the battery is carried out outdoors, rainwater or the like makes the operation dangerous in some cases.
In order to solve the problem described above, the other method has been proposed. In accordance with the other method, two coils are exposed to each other so as to allow power to be transferred from one of the coils to the other in a noncontact way also referred to as a wireless way. In this way, it is possible to carry out a noncontact electrical charging operation for electrically charging the battery without electrical contact between metallic members of a wireless electrical charging system. The proposed method has been adopted for the purpose of testing the method.
This wireless transfer of power is briefly explained as follows.
In recent years, a wireless electric-charge supplying operation requiring no cable and a wireless electric-charge supplying system for carrying out such a wireless supplying operation have been attracting attention. The wireless electric-charge supplying operation is carried out by adoption of a cableless method referred to as a magnetic-field resonance method making use of an electromagnetic resonance phenomenon.
At the present day, in the contemporary non-contact power transferring system adopting the already widely used magnetic induction method, it is necessary to share magnetic fluxes between a power supplier on the power supplying side and a power receiver on the power receiving side so that, in order to transfer power from the power supplier to the power receiver with a high degree of efficiency, it is necessary to position the power receiver at a location very close to the power supplier. In addition, when the power receiver is coupled to the power supplier, it is important to align the axis of the power receiver to the axis of the power supplier.
On the other hand, the non-contact power transferring system making use of the electromagnetic resonance phenomenon offers a merit that the principle of the electromagnetic resonance phenomenon allows power to be transferred over a distance longer than the distance between the power supplier and the power receiver in the non-contact power transferring system adopting the magnetic induction method. In addition, the non-contact power transferring system making use of the electromagnetic resonance phenomenon also offers another merit that the efficiency of the power transfer does not decrease much even if the axis of the power receiver is not aligned to the axis of the power supplier to a certain degree.
It is to be noted that, in addition to the magnetic-field resonance method, an electric-field resonance method also makes use of the electromagnetic resonance phenomenon.
In addition, in recent years, there has been reported a wireless power transferring technology for transferring a power of 60 W over a distance of 2 m by adoption of a magnetic-field resonance method which makes use of a magnetic-field resonance phenomenon.
On top of that, there has also been reported development of an efficient wireless power transferring system for driving electronic apparatus separated from each other by a distance of 50 cm in order to transfer a power of 60 W by adoption of a magnetic-field resonance method.
As described above, the wireless power supplying (or transferring) system of the magnetic-field resonance type is the same as the wireless power supplying (or transferring) system of the electromagnetic induction type in that, in both the systems, power is transferred by making use of a magnetic field. In the case of the wireless power supplying (or transferring) system of the magnetic-field resonance type, however, the electromagnetic resonance phenomenon is used in order to obtain a power-transfer distance about 10 times the power-transfer distance of the wireless power supplying (or transferring) system of the electromagnetic induction type.
FIG. 1 is a diagram showing a typical configuration of a wireless electrical charging system for electrically charging generally the battery of an electric car by adoption of a wireless power transferring technique.
As shown in the figure, the wireless electrical charging system 1 is configured to include a wireless electrical charging stand 2 and the electric car 3.
The wireless electrical charging stand 2 has a power-supply unit 21 and a power transferring coil 22.
On the other hand, the electric car 3 includes a power receiving coil 31, an AC (Alternating Current)-DC (Direct Current) conversion circuit 32 and a battery 33.
In the wireless electrical charging system 1, in general, the power transferring coil 22 also referred to as a power supplying coil is installed on the surface of the ground whereas the power receiving coil 31 is incorporated on the bottom of a vehicle which is the electric car 3 shown in the diagram.
In the case of a big-size vehicle such as a truck or a bus, however, as shown in a diagram serving as FIG. 2, the bottom of the vehicle is far from the surface of the ground. Thus, the distance between the power transferring coil 22 and the power receiving coil 31 is undesirably long. As a result, the efficiency of the power transfer between the power transferring coil 22 and the power receiving coil 31 unavoidably becomes extremely poor.
There have been proposed a variety of technologies for solving the problems described above, for example, referring to Japanese Patent Laid-open Nos. 2005-255144 (as Patent Document 1), 2005-269687 (as Patent document 2), 2000-152512 (as Patent Document 3), and 2000-092615 (as Patent Document 4).
Patent Documents 1 to 4 disclose methods each adopted for moving the power transferring coil 22 to approach the power receiving coil 31 by mechanically raising the power transferring coil 22 from the surface of the ground in order to take the power transferring coil 22 to a position closer to the power receiving coil 31.